We plan to continue our investigations on the capacity of several analogues of 1-chloro, 2,4-dinitrobenzene (DNCB) to induce tolerance to DNCB when fed to guinea pigs before attempted sensitization. These compounds would include non-sensitizers, weak sensitizers and strong sensitizers, such as 1,3,5-trinitrobenzene (poor sensitizer), 1,2,4,-trinitrobenzene (strong sensitizer) and 1,4-dichloro-2-nitrobenzene (non-sensitizer). Where tolerogenic activity can be demonstrated by our usual technic of feeding 60 mg of the 'tolerogen', titrations will be done to determine the endpoint of such activity. Where tolerogenic activity cannot be demonstrated by the usual technic, repeated feedings will be done in an attempt to induce tolerance. Where tolerance can be induced, studies will be carried out to determine its persistence. Attempts will be made to desensitize DNCB-sensitive guinea pigs by treatment with non-sensitizing or poorly sensitizing analogues of DNCB. We shall continue our cooperative studies with Dr. I. Silberberg on the role of the Langerhans cell in cell-mediated immunologic reactions.